Surface treatment method and device therefor

ABSTRACT

A surface treatment method wherein one or more active particle streams are generated and aimed at a surface to be treated so that the particle stream interacts therewith. The active particle stream consists of activated particles forming chemically active sites on the surface, and modifying particles occupying said sites. The energy of the activated particles is greater than the energy at break of the inhibited surface bonds of the surface, and lower than the radiative flaw formation energy on the surface. The strength of the particle stream at the treated surface is greater than a quantity N/t where N is the surface density of the inhibited bonds to be broken and t is the duration of exposure of any point on the treated surface to the stream. A device for carrying out the method is also provided.

[0001] The invention relates to a surface treatment method, inparticular for the surfaces of hard bodies and condensed media. Itlikewise relates to a device for implementing the method.

[0002] The method and device according to the invention may be used inparticular in the electrical engineering sector, in electronics, thechemical industry, the foodstuffs industry, machine tools, medicine,pharmaceuticals, and in particular for operations involving cleaning,sterilisation, pickling, the deposition of films, surface alloys, etc.

[0003] A treatment method is known for solid surfaces by means of plasmaunder vacuum, in the course of which a stream of activated particles isformed at low pressure. This method is described in the document “Theiono-plasmic treatment of materials”, Ivanovskii G. F. and Petrov V. I.,Moscow 1986. This particle stream acts on the surface of the solid bodyand the volatile products resulting from the interaction of the plasmaare evacuated from the reaction zone. The composition of the plasma isselected on the basis of the intention behind the treatment. In order toincrease the effectiveness of the operation by plasma under vacuum, thekinetic energy of the particles is increased up to 100 eV and above,which causes the appearance of flaws due to the destructive bombardmentsof the structure of the surface being treated. This treatment, whichrequires the maintaining of the vacuum, is used for preference insituations in which the surfaces to be treated are of restricteddimensions, in particular in the electronics industry.

[0004] A dynamic plasma treatment method (Dynamic Plasma Operation orDPO) is likewise known. This method is described in the documents“Dynamical Plasma Operating (DPO) of solid surfaces plasma jets”, KoulikP. P., ed. Solonenko O. P., Fedorchenko A. V., Frunze VSP 1990, pp.639-653, and “Dynamical Plasma Operating (DPO) of solid surfaces plasmajets”, Koulik P. P. et al., ed. Nouka, 1987, pp. 4-13, 58-96. Thisconsists of creating a hydrodynamic stream of activated particles,directing them towards the surface to be treated, causing them to reactwith this surface, and evacuating the volatile products accompanying thereaction, making use of the hydrodynamic stream pressure; i.e. at highpressure. The surface treated by this method is subject to the action ofhigh-enthalpy stream, which contain active particles (pickling,cleaning, depositing of films, etc.).

[0005] The approximate values of the parameters of the plasma stream areas follows:

[0006] Temperature: 8·10³-15·10³ K

[0007] Speed: 100-200 m/s

[0008] Density of heat flow on the surface, oriented perpendicular tothe stream:

[0009] 10²⁴-10²⁵ particles/m².s

[0010] For these plasma stream parameter values the interaction of theplasma with the surface can only be momentary, and does not exceed 10ms. The treated object passes through the plasma stream at a controlledspeed (approx. 1 m/s) in such a way that, for transient heat transfer,the maximum surface temperature does not exceed a given limit. Accordingto the technological operation concerned, this temperature level may beseveral tens of degrees, or even several hundreds of degrees.

[0011] Since the duration of the DPO method is less than thecharacteristic diffusion time in the solid body, the DPO treatment doesnot induce flaws in the structure of the solid body treated.

[0012] Since the temperature of the electrons on the surface of the bodyor medium being treated does not exceed 0.08 eV, and that of the ions,atoms, and other heavy components does not exceed 0.03 eV, it followsthat radiative flaws of the structure of the surface layer treated willbe excluded.

[0013] The principal physical energy of the DPO method is expressed bythe two inequalities:

l_(r)<<d<<l_(in)  (1)

[0014] where l_(r) is the mean length of the free path of the plasmaparticles, d is the thickness of the limit layer at the critical point,and l_(in) is the mean diffusion length of the activated particles ofthe plasma. The left inequality ensures the continuity of the plasmamedium, and that on the right the maintaining of imbalance necessary foran effective plasmochemical action on the surface.

[0015] The DPO method is easy to use, in particular at atmosphericpressure. Passage at atmospheric pressure allows for the productivity ofthe treatment to be enhanced by 10 to 100 times by comparison with thevacuum plasma process. The quality of the treatment is greater and thetechnology simplified.

[0016] Nevertheless, the DPO method can only be used under the followingconditions:

[0017] The treatment is applied by a high-enthalpy plasma jet;

[0018] Stationary cooling of the objects treated is excluded, and it isnecessary to work under non-stationary heat exchange conditions; i.e. tosubmit the treated surface to a momentary plasma action.

[0019] These conditions perceptibly restrict the possibilities of thistechnology, and render the treatment of many materials difficult.

[0020] The objective of the invention is to broaden the choice ofmaterials which can be treated, to make new surface treatments possiblethanks to the separate monitoring of the activated and stream-modifyingfunctions, and also to broaden the ranges of the physical parameters ofthe streams used for the treatment.

[0021] To this end, the invention relates to a surface treatment method,in particular of the surfaces of hard bodies and condensed media, in thecourse of which a stream or streams of activated particles is or arecreated, and directed on to the surface to be treated, and the particlestreams are caused to interact with the surface, and in which thestream(s) of activated particles is (are) composed of activatedparticles, forming chemically active sites on the surface, and modifyingparticles occupying these sites, the energy of the activated particlesbeing greater than the energy at break of the inhibited surface bonds ofthe treated surface, and lower than the radiative flaw formation energyon the surface, the intensity, at the level of the surface treated, ofthe stream of activated particles and the stream of the modifyingparticles being greater than the value N/t, where N is the surfacedensity of the inhibited bonds to be broken, and t is the presence timeof any point on the treated surface, under the stream.

[0022] The invention likewise concerns a device for surface treatmentfor the implementation of the method, comprising a device forintroducing the active product from an active particle stream generator,including an energy source, a reactor creating the stream of activatedparticles, a medium for transporting the activated particles generatedto the treated surface, and a medium for evacuating the residue productsof the treatment, and in which the active particle generator comprises:

[0023] a) Two reactors, the first creating a stream of activatedparticles, which, when coming in contact with the treated surface, formchemically active sites, the second creating a stream of modifyingparticles which have just occupied the sites;

[0024] b) Two transport devices for the activated and modifyingparticles respectively by means of flows carrying the activated andmodifying particles onto the treated surface, the duration of transportof the activated and modifying particles from the reactor to the surfaceto be treated being less than their respective life time durations;

[0025] c) A device for the relative displacement of the surface treatedin relation to the streams of activated and modifying particles,ensuring that the time lapse between the activation and modificationactions of one common area on the treated surface is less than the lifetime of the activated sites created by the activated particles, and thatthe areas of the surface to be treated, being first in contact with thestream of activated particles and then only with the stream of modifyingparticles;

[0026] d) Two evacuation devices, one for the activated particlesdeactivated after their action on the treated surface, the other for theresidual particles appearing after the action of modification of thesurface, the two systems being conceived in such a way that theevacuation of the one group will not cause any obstacle to the action orevacuation of the others.

[0027] According to one embodiment, the activated particles coincidewith the modifying particles, which allows the generator device to bereduced to an introduction device, a reactor, a transport device, adevice for displacement relative to the surface, and an evacuationdevice.

[0028] According to a variant embodiment, the reactor, the transport andevacuation devices may be common for the activated particles and themodifying particles.

[0029] According to another variant embodiment, the evacuation devicemay be common for the activated particles and for the residual products.

[0030] A description is provided below of the method and deviceaccording to the invention, making reference to the appended drawings,in which:

[0031]FIG. 1 is a general diagram illustrating the method;

[0032]FIG. 2 is a first embodiment example of the device according tothe invention; the reference figures relating to FIG. 2 are as follows:

[0033] 1. Argon plasma reactor for the excitation of nitrogen molecules(P=1 atm)

[0034] 2. Plasma pipe and jet containing excited nitrogen molecules(transport of activated particles)

[0035] 3. Heaters for di-tertiary butyl ((H₃ C)₃ C Mg C (CH₃)₃) vapourdiluted in a stream of neutral gas (Ar). Heating temperature: 1000 K

[0036] 4. Distribution pipe (transport of modifying particles Pg)

[0037] 5. Polyethylene surface to be treated

[0038] 6. Transport device providing for the relative movement of thesurface to be treated in relation to the two reactors. V= 5 m/s b=0.1 m

[0039] Exposure time for activation 2·10−3 s

[0040] 7. Evacuation device for the residual activation products

[0041] 8. Modification ventilation system

[0042] 9. Energy source.

[0043]FIG. 3 is a second embodiment example of the device according tothe invention; the reference figures relating to FIG. 3 are as follows:

[0044] 1. Reactor for heating and decomposition of the carbontetrafluoride (T>2000 K), with liberation of F atoms (activated andmodifying particles coinciding)

[0045] 2. Argon flow, transporting the F atoms towards the siliconsurface

[0046] 3. Silicon plate

[0047] 4. Device for moving the silicon plates under the F jet

[0048] 5. Ventilation (evacuation of residual particles)

[0049] 6. Energy source

[0050]FIG. 4 is a third embodiment example of the device according tothe invention; the reference numbers relating to FIG. 4 are as follows:

[0051] 1. Reactor, for heating and activating the oxygen and nitrogenmolecules in the air at T approx. 1000 K

[0052] 2. Pipe forming the jet of excited air and directing it towardsthe iridium surface

[0053] 3. Iridium object

[0054] 4. Device for transporting the iridium object at V approx. 1 m/s

[0055] 5. Ventilation (device for evacuating the residual products)

[0056] 6. Energy source

[0057] In order to resolve the problems of the method of the prior art,the solution consists of creating one or more streams of particles,directing them towards the treated surface, and causing these particlesto interact with the surface in such a way that, according to theinvention:

[0058] The stream of activated particles is formed by particles which,once activated by the plasma, form chemically active sites on thesurface,

[0059] The stream of modifying particles interacts with the surface, insuch a way as to occupy these active sites,

[0060] To do this, the energy of the activated particles is greater thanthe energy for breaking the inhibited bonds of the treated surface, andlower than the radiative flaw formation energy,

[0061] And the intensity of the stream of activated particles on thetreated surface, as well as of the stream of modifying particles, isgreater than the quantity N/t, where N is the surface density of theinhibited bonds of the treated surface, and t is the presence time of apoint of the treated surface under the stream.

[0062] The essence of the invention rests in the fact that, by contrastwith the DPO method, one or more streams are created which containspecially chosen particles, one group activated, the other modifying,the functions of which are different:

[0063] a) The role of the stream of activated particles is to transformthe treated surface into an activated state, i.e. to create chemicallyactive sites thanks to the breaking of the inhibited surface bonds; forexample:

[0064] surface bonds which are created as a result of the reconstruction(dimerisation and others) of the surfaces of silicon, gallium arsenic,and other homopolar and heteropolar semi-conductors, of gold, nickel,stainless steel, and other metals and alloys which do not oxidise;

[0065] oxide bonds for aluminium and other oxidisable metals,

[0066] lateral bonds of radicals in the polymers and bipolymers.

[0067] In order for activated particles to be able to play their part,their energy Ea must be greater than the energy at break of theinhibited bond. The energy Ea must, on the other hand, be lower than theradiative flaw formative energy in order to maintain the quality andstructure of the surface layer of the treated body.

[0068] As the intensity of the stream of activated particles drops, thetreated surface must be greater than the quantity N/t, where N is thesurface density of the inhibited bonds to be broken, and t is theduration of the presence of any point of the treated surface beneath thestream. The quantity N may be lower than the total number of theinhibited bonds of the treated surface in the event that the desiredtreatment does not involve the breaking of all the inhibited bonds ofthe surface.

[0069] b) The role of the stream of modifying particles is to occupy thechemically active sites, resulting of the impact of the activatedparticles; i.e. to make use of the activated state of the surfacecreated by the activated particles, in order to operate the desiredtreatment in an effective manner, such as, for example, the depositingof films (with optimum chemical adhesion), pickling (with the formationof volatile, chemically solid molecules), etc.

[0070] The processes of formation and bonding of the chemically activesites of the treated surface can be developed in parallel. The modifyingparticles can be used in an active state.

[0071] The intensity of the stream of modifying particles on the treatedsurface is selected to be greater than the quantity N/t, in such a wayas to make use of all the activated sites created by the activatedparticles. In this case, the treatment is optimum.

[0072] In certain particular cases, the activated and modifyingparticles may coincide, being of the same chemical nature.

[0073] The physico-chemical mechanism of the surface treatmentaccordingly consists of the following:

[0074] In the first instance, under the action of the activatedparticles, the atoms of the surface acquire chemical properties whichcould have been obtained, for example, by suddenly removing a surfacelayer of macroscopic thickness of the material which is to be treated.It is this layer which, under usual conditions, presents an obstacle tothe appearance of a chemical activity of the surface.

[0075] Accordingly, the effect of the action of the activated particlesis to create chemically active sites on the surface (radicals).

[0076] Next, the modifying particles bond with these sites, which allowsfor the desired surface treatment to be carried out in an effectivemanner. Because the activation state of the surface has a limited lifetine, the process of bonding with these sites must take place within ashorter time than this life time.

[0077] This description of the physico-chemical mechanism for thesurface treatment allows for provision to be made for the possibility(and even the necessity) of separate formation of the streams ofactivated particles and modifying particles. This allows for a check tobe conducted separately on the activating and modifying functions of thestream.

[0078] The particles can be activated by different means, notnecessarily by the heating of the gas which they form up to atemperature of (10 to 15)·10³K and the formation of a plasma, a is thecase with the DPO method. In the present invention, the activation ofthe particles and the heating of the gas which they form are processeswhich are inherently distinct and independent. It is therefore possibleto monitor the heating, i.e. the thermal function of the stream,separately, in accordance with the demands imposed on the treatmentdesired.

[0079] The particles can be activated, for example by radiation or as aresult of collisions with a stream of charged particles, accelerated inan electromagnetic field. It is also possible to monitor the streams ofactivated and modifying particles by rapidly reducing or increasing thepressure of the gas which they form.

[0080] The range of the physical parameters of the stream acting on thetreated surface can therefore be very substantially enlarged andenriched.

[0081] The density of the stream of particles falling on the treatedsurface must be reasonably great, without the activated sites being ableto relax spontaneously, i.e. to revert to their original state beforethe modifying particles have fixed them. A high density of the flow canbe achieved in two ways. The first consists of increasing the speed ofthe particles, and therefore their kinetic energy, in the directionperpendicular to the treated surface. This method is however limited tothe formation of the radiative flaws in the structured treated.

[0082] The second method consists of increasing the density of thestream particles. An increase in density leads to the transition of thestream, which passes from the molecular state into a viscous stream. Inthis case, between the falling stream and the treated surface, a limitlayer is formed, the thickness of which can be varied in order to varythe intensity of the treatment desired.

[0083] The limit layer plays a triple role in the present invention.

[0084] 1) It plays the part of a barrier for the activated particleswhich, in order to reach the treated surface, must be diffused acrossthe limit layer. The following condition is accordingly imposed on thethickness of the limit layer:

d<Ö 6D t_(in)  (2)

[0085] where D is the coefficient of diffusion and t_(in) is the lifetime of the activated particles. The hydrodynamic qualities of thestream are accordingly chosen in such a way that the inequality (2) issatisfied. This latter inequality replaces the right-hand inequality in(1).

[0086] 2) The limit layer is a useful barrier against the return andredepositing on the treated surface of the residual molecules resultingfrom the treatment of the surface (during pickling, for example).

[0087] 3) The limit layer may play an active role, generating activatedparticles. In this case, the hydrodynamic parameters of the stream areselected in such a way as to satisfy the inequality:

[0088]  d₁<Ö 6D t_(in)

[0089]

[0090] where d₁ is the distance of the treated surface from thegeneration area of the activated particles.

[0091] It is evident that d₁<d, a case of which (1) does not takeaccount.

[0092] The invention allows for the realisation, apart from knowntechnologies which are implemented by the DPO method, of technologieswhich have not yet been implemented: the adhesive bonding or welding atthe molecular level of pairs of uniform or heterogenous materials, whichcannot be bond or welded to each other by the known methods, thetreatment of non-regular, particularly natural, organic polycondensates,which are obtained by guiding the activating and modifying thermalfunctions of the stream and broadening the range of physical parametersof the stream used for the treatment.

[0093] The present invention can be used in particular for thedisinfection and sterilisation of surfaces, for film depositionoperations, for the pickling and cleaning of surface alloys in thesemi-conductor industry, in order to create bactericidal surfaces,coatings for a variety of purposes, etc.

[0094] The originality of the process according to the invention restsin the performance of the following procedures:

[0095] 1 creation of several streams of activated particles,

[0096] 2 application of these streams on the surface to be treated insuch a way that the activated particles of the streams acting on thesurface create chemically active sites, and that the modifying particlesoccupy these sites,

[0097] 3 this operation is made possible thanks to the provision for theactivated particles of a greater energy than the energy at break of theinhibited bonds of the surface to be treated, and lower than theradiative flaw formation energy, and

[0098] 4 the intensity on the treated surface of the stream of activatedparticles, as well as of the stream of modifying particles is largerthan the quantity N/t, where N is the surface density of the inhibitedbonds to be broken and t is the duration of presence of any point on thesurface beneath the stream.

[0099] It will also be noted that the process according to the inventiondiffers from the DPO method due to the fact that:

[0100] 1 the active particle stream is formed of activated particles,creating chemically active sites on the surface, and the stream ofmodifying particles occupies these sites,

[0101] 2 this is with an energy of the activated particles greater thanenergy at break of the inhibited bonds of the surface and lower than theradiative flaw formation energy, and

[0102] 3 the intensity on the treated surface of the stream of activatedparticles, as well as of the modifying particles, is greater than thequantity N/t, where N is the surface density of the inhibited bonds atbreak and t is the duration of the presence of any point on the treatedsurface beneath the stream.

[0103] The process according to the invention and the device for itsimplementation may be put into effect on equipment of different types.Given that this process allows for highly diverse surface treatments tobe carried out, only the essential elements will be cited which arenecessary for any equipment for the realisation of the invention: Astream generator for the activated particles, a stream generator formodifying particles (the two generators, in certain specific cases, maybe reduced to one single generator), devices for bringing about thecontact between the particles streams and the treated surface, transportdevices which ensure the relative movement of the treated surface andthe activating and modifying flows, devices for the evacuation of theresidual products of the process. The general layout of the process isshown in FIG. 1.

[0104] Some embodiments of the devices for the implementation of theprocess are indicated below, in relation to FIGS. 2 to 4.

EXAMPLE 1 Deposition of Magnesium on Polyethylene

[0105] The energy of the inhibited bonds is equal to 4 eV, the surfacedensity of these bonds is 10¹⁵ cm⁻², and the part of the surface bondsto be broken is 10%. In view of the thermal instability of polyethylene,the duration of treatment must not exceed 10⁻³ s. The density of thestream of activated particles must therefore not be less than 10¹⁷ cm⁻²s⁻¹. For the quality of activated particles, excited molecules ofnitrogen N₂ are chosen (B³ Pg, A³S+_(a), aS−u, a¹Pg) with activationenergy values within the limits from 6 to 9 eV. With regard to thequality of the modifying particles, magnesium atoms are chosen, obtainedthanks to the thermal decomposition of di-tertiary butyl vapours ((H₃C)₃ C Mg C (CH₃)₃) diluted in a stream of neutral gas heated to 1000 K.The density of the stream of modifying particles (magnesium atoms) mustbe greater than 10¹⁷ cm⁻² s⁻¹.

[0106] The result is that the polyethylene is covered by a layer ofmagnesium with an adhesion energy of the order of 4 eV and a partialitycoefficient of 0.1. This device is illustrated in FIG. 2.

EXAMPLE 2 Cold Pickling on Silicon

[0107] The energy of the inhibited bonds of silicon is about 1 eV; thetotal surface density of these bonds is about 10¹⁵ cm⁻², and theproportion of the surface bonds at break is 100%. To obtain a picklingrate of 10 microns/s, the time t of presence beneath the stream for anatomic layer should be about 3·10⁻³ s. Accordingly, the stream ofactivated particles must not be less than 3·10¹⁸ cm⁻² s⁻¹. For thequality of the activated particles, fluorine atoms are chosen,thermically obtained thanks to the decomposition of the carbontetrafluoride (CF₄) in the stream of gas heated to more than 2000 K. Theactivation energy in this case is the energy of the affinity of theelectrons to the fluoride atoms, and is 3.4 eV. With regard to themodifying particles, fluoride atoms are likewise used; in this example,the activated and modifying particles coincide.

[0108] The result is that the pickling of silicon is effected at a speedof 10 microns/s with energy losses of an order of magnitude lower thanthat of the DPO method. The corresponding device is illustrated in FIG.3.

EXAMPLE 3 Molecular Oxidation of Iridium

[0109] The energy of the inhibited bonds is equal to 0.1 eV, the totalsurface density of these bonds being 3·10¹⁵ cm⁻², the proportion of thesurface bonds at break being 100%. The time t is of the order of 10⁻² s.The stream of activated particles must not be lower than 3·10¹⁷ cm⁻²s⁻¹.

[0110] For the quality of the activated particles, molecules of oxygenand nitrogen from the heated air (about 1000 K) are chosen. Theactivation energy in this case is the thermal energy of the molecules.As modifying particles, oxygen molecules are used. The stream ofmodifying particles cannot be below 3·10¹⁷ cm⁻² s⁻¹. In this example,the real stream of the activated particles, considering the use of air,far exceeds the limit demands of the method.

[0111] The result is that the surface of the iridium is covered with amono-layer of oxygen molecules, easily decomposable. The energyexpenditure values are two orders of magnitude lower than those of theDPO method.

[0112] In this way, the process according to the invention allows forthe scope of utilisation of the DPO method to be broadened, and inparticular allows for the creation of new methods of treatment, such as,for example, the formation of solidifying metallic layers on polymers,while still reducing energy expenditure values, thanks in particular tothe reduction of the temperature of treatment. The corresponding deviceis illustrated in FIG. 4.

1. Surface treatment method, in particular for the surfaces of hardbodies and condensed media, in the course of which one or more streamsof active particles are created, these being directed onto the surfacewhich is to be treated, and caused to interact with the surface,characterised in that the stream(s) of active particles is/are composedof activated particles, forming at first chemically active sites on thesurface, and of modifying particles subsequently occupying these sites,the energy of the activated particles being greater than the energy atbreak of the inhibited surface bonds of the treated surface and lowerthan the radiative flaw formation energy on the surface, the intensity,at the level of the surface treated, of the stream of activatedparticles and of the stream of modifying particles being greater thanthe quantity N/t, where N is the surface density of the inhibited bondsto be broken, and t is the presence time of any point on the treatedsurface beneath the stream.
 2. Device for the implementation of themethod according to claim 1 , characterised in that it comprises asupport device for the surface to be treated, a first generator forcreating a stream of activated particles, a device for introducingparticles to be activated, a second generator for creating a stream ofmodifying particles, a device for introducing chemical compoundsprecursors of modifying particles, said first generator comprising atransport device of the activated particles towards the surface to betreated with a sufficient speed to allow the formation of chemicallyactive sites on said surface, said second generator comprising atransport device of the modifying particles on said surface comprisingchemically active sites and at least an evacuation device for theresidual products.
 3. Device according to claim 2 , characterised inthat said support device for the surface to be treated is subject to arelative displacement in relation to the streams of activated andmodifying particles, ensuring that a same zone of the surface to treatedis at first under the stream of the activated particles and subsequentlyunder the stream of the modifying particles, so that the time lapsebetween the actions of activation and modification is lower than thelife time of the chemically active sites created on the surface by theactivated particles.
 4. Device according to claim 2 , characterised inthat said first and second generators are replaced by only one generatorensuring the simultaneous generation of a stream of activated particlesand a stream of modifying particles.
 5. Device according to claim 4 ,characterised in that said device for introducing particles to beactivated and said device for introducing chemical compounds precursorsof modifying particles consist in a single introduction device ensuringthe simultaneous introduction of the particles to be activated and thechemical compounds precursors of modifying particles and in that thegenerator allows the creation of a single stream containing bothactivated particles and modifying particles.
 6. Device according toanyone of claims 2 to 5 , characterised in that said transport devicesof the activated particles on the one part and of the modifyingparticles on the other part, consist in gas streams and in that thespeed of said streams is such that the transport durations arerespectively lower than the life times of said activated and modifyingparticles.
 7. Device according to claim 2 , characterised in that aparticles evacuation device ensuring the evacuation of the activatedparticles, when totally or partially deactivated after their interactionon the surface to be treated, is associated to the first generator, andin that a particles evacuation device ensuring the evacuation of theresidual particles resulting from the interaction of the modifyingparticles with the surface comprising chemical active sites isassociated to the second generator.